Integrated photonic devices fabricated in a high refractive index contrast material system such as, for example, a Si/SiO2 material system are very sensitive to small variations in the device dimensions. Such devices are very sensitive to small variations in the thickness t and etch depth d, as well as to small variations in the linewidth w of the device structures made in the core material (e.g., Si) of the material system. FIG. 1 shows a cross section of an example photonic structure, illustrating thickness t, linewidth w and etch depth d. For example, a variation of 1 nanometer (nm) in the linewidth w can result in a shift in spectral response of wavelength selective photonic devices (such as e.g. ring resonators) on the order of 1 nm or 2 nm. Variations in the linewidth w and etch depth d of device structures may be mainly influenced by the patterning process, including photolithography and etching. Variations in the thickness t or height of the device structures may be determined by the substrate manufacturing process.
For example, integrated photonic devices can be fabricated in thin silicon-on-insulator (SOI) substrates (comprising e.g. a 220 nm thick silicon device layer on top of a 2000 nm thick silicon dioxide layer). The thickness variation of the silicon device layer of a silicon-on-insulator substrate may depend on the vendor specification. The thickness variation is typically 10% over the wafer or substrate. For a 220 nm thick silicon device layer, this means a variation of about 22 nm in thickness of the silicon device layer over the substrate. This may correspond to a shift or variation of about 22 nm to 44 nm in the spectral response of wavelength selective optical devices over the wafer. This shift or variation is unacceptable for commercial viable silicon photonic technology.
In U.S. Pat. No. 6,537,606, a method is described for improving the thickness uniformity and reducing the surface roughness of thin films such as the top silicon film of a silicon-on-insulator wafer. The method uses a vacuum GCIB (Gas Cluster Ion Beam) etching and smoothing process. After initial thickness non-uniformity characterization of the top silicon layer, the film thickness map information is fed into the GCIB beam-control apparatus as a data file. Based on a previously measured beam removal function and a previously measured relationship between etch rate and dose for a particular set of GCIB parameters, a mathematical algorithm is used to create a beam-dose contour to selectively remove surface material and thereby achieve a uniform film thickness.
Overview
Example embodiments of the present disclosure provide methods for improving the uniformity of the spectral response of photonic devices over a wafer or substrate and for improving the repeatability of the spectral response of photonic devices from wafer to wafer and from batch to batch. The methods according to the present disclosure can in particular advantageously be used for integrated photonic devices fabricated in a high refractive index contrast material system.
Particular aspects of the disclosure are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims as appropriate and not merely as explicitly set out in the claims.
The present disclosure provides a method for improving the uniformity and repeatability of the spectral response of photonic devices fabricated in a thin device layer such as a silicon device layer of an SOI substrate, over a predetermined substrate area. In an example, the method comprises: (i) establishing an initial device layer thickness map for the predetermined area; (ii) establishing a linewidth map for the predetermined area; (iii) establishing an etch depth map for the predetermined area; (iv) based on the initial device layer thickness map, the linewidth map and the etch depth map, calculating an optimal device layer thickness map and a corresponding thickness correction map for the predetermined substrate area taking into account device design data; and (v) performing a location specific corrective etch process in accordance with the thickness correction map.
In an example method according to the present disclosure, establishing an initial device layer thickness map comprises measuring the initial thickness of the device layer over the predetermined substrate area as a function of x and y, where x and y are the spatial wafer coordinates. For example, the initial device layer thickness can be measured by means of spectroscopic ellipsometry, e.g. with an accuracy better than 1 nm. In an example, the number of measurement points per wafer can for example be on the order of 300 to 400 points for a 200 mm wafer. However, the present disclosure is not limited thereto, and the number of measurement points per unit area can be higher or lower.
In an example method according to the present disclosure, establishing a linewidth map and establishing an etch depth map may comprise measuring the linewidth and etch depth over the predetermined substrate area as a function of x and y, where x and y are the spatial wafer coordinates. Measuring the linewidth and etch depth may, for example, be done by means of scatterometry.
In an example method according to the present disclosure, calculating an optimum device layer thickness map may comprise calculating as a function of the spatial wafer coordinates an optimum device layer thickness needed for matching a predetermined spectral response of a photonic device. The thickness correction map may be determined by calculating the difference between the optimum device layer thickness and the initial device layer thickness as a function of the spatial wafer coordinates. This difference corresponds to the thickness correction to be performed by the location specific corrective etch process.
In example embodiments of the present disclosure, the location specific corrective etch process may for example be a Gas Cluster Ion Beam process, as for example described in U.S. Pat. No. 6,537,606.
The method of the present disclosure may be particularly relevant and advantageous for integrated photonic devices fabricated in a high refractive index contrast material system, i.e. a material system having a high refractive index contrast between a waveguide core material and a cladding material (difference in refractive index between the core material and the cladding material larger than 1). Photonic devices fabricated in such material systems are highly sensitive to process variations. Examples of such high refractive index contrast material systems are Si/SiO2, SiN/SiO2, SiON/SiO2, TaO2/SiO2, Si(O)C/SiO2, InGaAsP/SiO2 and Ge/Al2O3. Other examples are possible as well.
In a first aspect of the present disclosure, the thickness correction map may be established before starting the fabrication of photonic device structures, and the location specific corrective etch process is performed before fabrication of the photonic device structures. Establishing the initial device layer thickness map may comprise measuring the initial thickness of the device layer on the non-processed substrate as a function of the spatial wafer coordinates in the predetermined area. Establishing a linewidth map and establishing an etch depth map may comprise estimating the linewidth and etch depth as a function of the spatial wafer coordinates in the predetermined area, based on data previously collected from process control.
In a second aspect of the present disclosure, the thickness correction map may be established after fabricating the photonic device structures. Establishing the initial device layer thickness map may comprise measuring the thickness of the device layer on the processed substrate as a function of the spatial wafer coordinates in the predetermined area. Establishing a linewidth map and establishing an etch depth map may comprise measuring the linewidth and etch depth as a function of the spatial wafer coordinates in the predetermined area after fabrication of the photonic devices. In this second aspect, additionally the optical response of the photonic devices can be measured and this optical response (such as bandwidth, maximum wavelength, resonance wavelength, and other optical responses) of the devices can be used as an additional input for calculating the thickness correction map.
In example embodiments of the present disclosure a thickness correction can be performed before fabricating the photonic device structures, according to the first aspect. In addition, after fabrication of the photonic device structures a thickness correction may be performed according to the second aspect.
For purposes of summarizing the disclosure and the advantages achieved over the prior art, certain objects and advantages of the disclosure have been described herein above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the disclosure. Thus, for example, those skilled in the art will recognize that embodiments of the present disclosure may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein. Further, it is understood that this overview is merely an example and is not intended to limit the scope of the invention as claimed. The invention as recited in the claims, both as to organization and method of operation, together with features and advantages thereof, may best be understood by reference to the following detailed description when read in conjunction with the accompanying drawings.